a. The Need For Delivery Of Gas At A Constant Known Flow Rate
There are many situations in which it is required that gas be delivered through a line at a constant and known flow rate. Various types of chromatography apparatus have this requirement.
Other examples may be seen in the methods and apparatuses disclosed in Clingman U.S. Pat. Nos. 3,777,562; 4,062,236; 4,125,018; and 4,125,123. Generally speaking, these methods and equipment measure the calorific value of a gaseous fuel by causing the fuel to be delivered to a combustion system at a flow rate which maximizes the adiabatic flame temperature. The volumetric flow rate at ambient conditions must be accurately determined in these methods and equipment because it is a component in the determination of calorific value. The measurement of volumetric flow rate must be made independently of temperature, pressure, and gas compositions. The last of these requirements, independence from gas composition, is particularly important in such methods and equipment, because they are employed in situations where the gas composition can vary continually and is not known in advance of the making of the measurement. Thus the flowmeter, of whatever kind employed, cannot be calibrated for a particular gas composition, as is possible for some other gas flow measurement situations. The flow measurement must also be reproducible within one part per thousand.
b. Deficiencies of Prior Art Methods and Equipment
The conditions and requirements outlined above are not well met by the commonly used methods of flow measurement or control. Orifice meters and other methods which depend on measurement of the pressure drop across a flow restriction are sensitive to gas density, which is a function of temperature, pressure, and gas composition. Hot wire flow meters are sensitive to gas properties. Axial flow turbine flow meters are relatively free from sensitivity to gas composition, but contain moving parts which are subject to wear, causing maintenance problems and making recalibration work necessary at intervals as the wear occurs.
c. Certain Prior Art
U.S. Pat. No. 3,125,881 to Peters et al. shows a flow measurement system for liquid in which the time required for a predetermined volume of liquid to flow into a tank is measured to determine flow rate. Following each such measurement, the liquid is drawn from the tank more rapidly than it enters it, to empty the tank in readiness for the next measurement.
U.S. Pat. No. 3,500,826 to Haire controls the feed of gas from a high pressure supply to a low pressure receiver from which it is withdrawn at uneven rates (as in an oxygen mask receiver) by sensing the pressure in the receiver and utilizing the sensed value to control the setting of a valve between the supply and the receiver.
U.S. Pat. No. 3,878,376 to Sholes et al. utilizes a computer to direct the actuation of valves in high pressure, atmospheric pressure and vacuum lines connected to a closed pressured volume of gas to adjust the pressure therein in response to signals from a pressure transducer sensing the pressure in the volume.
U.S. Pat. No. 3,665,959, to Castillon shows a flow meter using a sonic nozzle so that advantage may be taken of the circumstance that flow rate through such a nozzle is solely a function of absolute temperature and pressure upstream of the nozzle, both of which parameters being measured to obtain the flow rate.
U.S. Pat. No. 3,633,416 to Van Dyke et al. and U.S. Pat. No. 4,096,746 to Wilson et al. shows systems utilizing diaphragm actuated devices to control pressure, flow, or both.
U.S. Pat. No. 3,970,472 to Jones et al. shows a flow measurement system using a rotometer type flowmeter in conjunction with an upstream regulator.
The following U.S. patents all show systems employing orifice type meters for measuring differential pressure across the meter: U.S. Pat. Nos. 2,132,338 to Ziebolz; 2,317,807 to Ryder; 2,352,312 to Donaldson; 2,862,162 to Baring; 2,878,825 to Grogan et al.; and 3,543,784 to Smith.